Summary

We noticed that there seems to have been some typos in the borderStrong SCSS tokens 🫠
This causes the variables to not actually be available for use.

Changes

• renames SCSS borderStrong tokens for Borealis dark
• adds missing !default flag (since we're here 🤷‍♀️)

QA

• verify the tokens are available:

  • checkout this PR
  • use the border tokens, e.g. $euiColorBorderStrongSuccess in our EUI docs, e.g. in packages/eui/src-docs/src/views/home/_home.scss to add a background color somewhere

  // example
  .guideSideNav {
    background-color: $euiColorBorderStrongSuccess !important;
  }